It sounds like your problem is mostly in other people's perception of you (rather than that you actually "feel" cold toward others). If that is the case, make a list of what you would consider a "warm" person?

When you decide something like, "appears cheerful, smiles at others, shows enjoyment of being with others by sincere compliments. . ." then you have tangible behaviors you can work to include in your own life? As it would be based on what you consider warm, it would not be inconsistent with you in any way; if you like to get smiles, you rightly figure others might like to receive smiles so adding more smiling to your behavior, literally teaching yourself to smile should have the benefit of getting you smiles in return so the whole interaction warms up and you start hearing, "You are so nice, chipperdear. I love when you first catch sight of me and beam me one of those warm smiles of yours!".